Field of the Invention
The present invention relates to mobile radio communications with focus on self optimizing networks and in particular mobility robustness optimization. Particularly, the present invention is concerned with so-called short stays, which are sometimes also referred to as “rapid” handover, where a UE is handed over from cell A to cell B, and cell B handovers the same user to another cell C shortly after.
Related Background Art
Prior art which is related to this technical field can e.g. be found in:
[1] 3GPP TS 36.300: “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description”, Stage 2;
[2] 3GPP TR 36.902: “Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Self-configuring and self-optimizing network (SON) use cases and solutions”, Technical Report;
[3] A. Catovic et al., “Handover failure messaging schemes,” US 2010/0173633 A1;
[4] A. Catovic et al., “Adaptation of handover parameters,” US 2010/0173626 A1; and
[5] 3GPP TS 36.423: “Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 application protocol (X2AP),” Technical Specification.
The following meanings for the abbreviations used in this specification apply:
3GPP 3rd generation partnership project
SON self optimizing networks
MRO mobility robustness optimization
RLF radio link failure
HO Handover
HOF handover failure
RCA root cause analysis
KPI key performance indicator
RAT radio access technology
UE user equipment
OAM operation, administration and maintenance
ECGI E-UTRAN cell global identification
E-UTRAN evolved UMTS terrestrial radio access network
UMTS universal mobile telecommunications system
PM performance management
NB node B
eNB evolved node B
IE information element
C-RNTI cell radio network temporary identity
ENUM enumeration
LTE long term evolution
RAN radio access network
RIM RAN information management
RNC radio network controller
BSC base station controller
Conventionally, optimization of network configuration parameters in 2G or 3G networks is based on labor- and cost-intensive drive testing. For a first roll-out, network-wide default configuration parameters are used and if performance management counters are accumulating RLFs or even call drops in certain service areas, several optimization loops with drive testing are started to adapt parameters in a cell-specific manner.
The target of MRO is to automate the optimization of those network configuration parameters which are triggering a handover and comprise e.g. event thresholds, timers, etc., such that first and foremost radio link failures and handover failures are reduced and secondly also unnecessary handovers like ping pongs or short stays are prevented. The MRO procedure consists of two phases:                root cause analysis phase, where all needed information to analyze a mobility problem is brought together to generate corresponding cell or cell-pair specific KPI statistics; and        correction phase, where based on the KPI statistics corresponding countermeasures are determined and measurement parameters triggering a HO are adjusted.        
Methods are provided for root cause analysis of RLF afflicted mobility problems, e.g. RLF reports from UE and inter-eNB information exchange, such as RLF indication and handover report message. However, these methods are not applicable for unnecessary handovers like ping pongs or short stays, since there is no RLF. Though an end user does not sustain a serious deterioration of the service quality, unnecessary handovers should be avoided since they result in increased network signaling load.
A ping pong handover means that a UE is handed over from cell A to cell B, and cell B handovers the same user back to cell A shortly after. Ping pong is a special case of the short stay problem where the second “rapid” handover goes back to cell A. The cell A is able to get aware of this “ping pong” problem by checking UE history information which is provided with an HO preparation message. The cell A is responsible for this problem, since it initiated the first unnecessary handover and is able to administer and increment a corresponding KPI counter without additional information exchange among nodes. However, in case of short stays where the UE is handed over further to a third cell C after being very shortly connected with cell B, the “guilty” cell A is not getting aware of this “short stay” problem automatically as in the ping pong case.
The problem is that there is no MRO mechanism in terms of root cause analysis which provides the information of a “short stay” problem to the responsible cell, i.e. there are no means for figuring out and informing the responsible cell, which should administer a corresponding performance counter or KPI.
Furthermore, there is also no proper specified MRO approach for short stay detection, including ping pong as special case, in case of cell individual short stay timing criteria, i.e. different thresholds determining a short stay for cell A and cell B.
As described above, only RLF afflicted mobility problems are considered so far from MRO perspective. The ping pongs are basically detectable without information exchange, since the UE is coming back to the “guilty” cell, i.e. that cell responsible for the problem and where a corresponding KPI should be counted.
The prior art documents [1] to [4] provide a description of methods and messaging applied to detect mobility problems including ping pong assuming a common network-wide timing criterion. However, the detection of short stays has not been considered so far and there is no proper mechanism to manage a corresponding problem detection in the responsible cell.